1. Field of the Invention
The present invention is related to a control system for controlling the feeding and burning of a pulverized fuel in a glass melting furnace and, more specifically to a control system that feeds pulverized fuel in a glass melting furnace.
2. Related Prior Art
Melting glass has been done for different kinds of furnaces as well as types of fuels, depending on the final characteristics of the product and also regarding the thermal efficiency of the melting and refining processes. Unit melter furnaces have been used to melt glass (by means of gas fuel), these furnaces have several burners along the sides of the furnace, the whole unit looks like a close box where there is a chimney that can be placed either in the beginning of the feeder or at the very end of the furnace, it means, going downstream. However there is an enormous heat loss in the glass leaving high-temperature operating furnaces. At 2500xc2x0 F., for example, the heat in the flue gases is 62 percent of the heat input for a natural gas fired furnace.
In order to take advantage of the remaining heat of the flue gases, a more sophisticated and expensive design come out, named as the regenerative furnace. It is well known that, in order to operate a regenerative glass melting furnace, a plurality of gas burners are associated with a pair of sealed regenerators disposed side-by-side. Each regenerator has a lower chamber, a refractory structure above the lower chamber and an upper chamber above the structure. Each regenerator has its corresponding port connected to the respective upper chamber with a melting and refining chamber of the furnace. The burners are arranged to burn fuel, such as natural gas, liquid petroleum, fuel oil or other gaseous or liquid fuels which are suitable for use in the glass melting furnace and thereby supply heat for melting and refining the glass making materials in the chamber. The melting and refining chamber is fed with glass making materials at one end thereof at which is located a doghouse and has a molten distributor disposed at the other end thereof, which comprises a series of ports through which molten glass may be removed from the melting and refining chamber.
The burners may be mounted in a number of possible configurations, for example a through-port configuration, a side-port configuration or an under-port configuration. Fuel, e.g. natural gas, is fed from the burner into the incoming stream of pre-heated air called xe2x80x9ccombustion airxe2x80x9d also, coming from each regenerator during the firing cycle or reversal sequence, and the resultant flame and products of combustion produced in that flame extend across the surface of the melting glass, and transfer heat to that glass in the melting and refining chamber.
In operation, the regenerators are cycled alternately between combustion air and exhaust heat cycles. Every 20 minutes, or 30 minutes, depending on the specific furnaces, the path of the flame is reversed. The objective of each regenerator is to store the exhausted heat, which allows a greater efficiency and a higher flame temperature that could not be achieved with normal ambient air.
For operating the glass melting furnace, the fuel is fed to the burners and, the combustion air supplied is controlled by measuring the airflow generated at the exit of the combustion fan and at the top of the structure, the, quantity of oxygen and combustible material present to ensure that within the melting chamber or at points along the melting chamber, the combustion air fed is less than that required for complete combustion of the fuel being supplied.
In the past, the fuel used to melt glass was fuel oil, coming from distillation of petroleum. For many years this kind of fuel was used, but the tighten of environmental regulations have been pushing for reduction of fuel oil, since this kind of oil has impurities coming from the petroleum crude oil, such as, sulphur, vanadium, nickel, and some other heavy metals. This kind of fuel oil produce pollutants such as SOx, NOx and particulates. Recently the glass industry has been using natural gas as a cleaner fuel. All the heavy metals and sulphur coming in the liquid stream of petroleum residuals from distillation are not contained in natural gas. However, the high temperature produced in the flame of natural gas has been very effective for producing more NOx than other pollutants. In this sense, a lot of effort has been done in order to develop low NOx burners for firing natural gas. Additionally, different technologies have been developed to prevent the NOx formation. An example of this is the Oxy-fuel Technology, which utilizes oxygen instead of air for the combustion process. This technology has the inconvenient of requiring a unit melter furnace with a special preparation of the refractories since air infiltration need to be prevented. The use of oxygen also produce a higher temperature flame, but in the absence of nitrogen the NOx production is drastically reduced.
Another inconvenient for the oxy-fuel process is the cost of the oxygen itself. In order to make it cheaper it needs to place an oxygen plant besides the furnace in order to feed the required oxygen by the melting process.
However, the continuing upward spiral of energy costs (primarily natural gas) have forced the major float glass manufacturers to add xe2x80x9csurchargesxe2x80x9d to truckloads of flat glass. Natural gas prices have increased over 120% this year (in Mexico only or elsewhere), far above previous estimates.
The general consensus among Glass Industry insiders is that Distributors will be forced to take a close look at these new xe2x80x98surchargesxe2x80x99, and most likely be forced to pass them along.
Taking into account the previous art, the present invention is related to apply different technologies to reduce the melting cost, using a solid fuel coming from the petroleum residuals of distillation towers, such as petroleum coke, in order to be used for glass production in an environmentally clean way
The main difference of this type of fuel regarding fuel oil and natural gas is the physical state of the matter, since fuel oil is a liquid phase, natural gas is a gas phase while petroleum coke for instance is a solid. Fuel oil and petroleum coke have the same kinds of impurities, since both of them are coming from residuals of distillation tower of crude oil. The significant difference is the amount of impurities contained in each of these. Petroleum coke is produced in three types of different processes called delayed, fluid and flexi. The residuals from the distillation process are placed in drums and then heated up to from 900xc2x0 to 1000xc2x0 Fahrenheit degrees for up to 36 hours in order to take out most of the remaining volatiles from the residuals. The volatiles are extracted from the top of the coking drums and the remaining material in the drums is a hard rock make if around 90 percent of carbon and the rest of all the impurities from the crude oil used. The rock is extracted from the drums using hydraulic drills and water pumps.
A typical composition of petroleum coke is given as follows carbon about 90%; hydrogen about 3%; nitrogen from about 2% to 4%; oxygen about 2%; sulphur from about 0.05% to 6%; and others about 1%.
Petroleum solid fuels have already been used in cement and steam power generation industries. According to the Pace Consultants Inc. the use of petroleum coke in the year 1999 for cement and power generation were between the 40% and 14% respectively.
In both industries, the burning of petroleum coke is used as a direct fire system, in which the atmosphere produced by the combustion of the fuel is in direct contact with the product. In the case of cement production, a rotary kiln is needed in order to provide a thermal profiled require by the product. In this rotary kiln, a shell of molten cement is always formed avoiding the direct contact of the combustion gases and flames with the refractories of the kiln, avoiding attack thereof. In this case, the calcined product (cement) absorbs the combustion gases, avoiding the erosive and abrasive effects of vanadium, SO3 and NOx in the rotary kiln.
However, due to the high sulfur content and vanadium the use of petroleum coke as fuel is not common in the glass industry, due to the negative effect on the structure of the refractories and in addition the environmental problems.
The glass industry use several kinds of refractory materials, and most of them are used to accomplish different functions, not only the thermal conditions but also the chemical resistance and mechanical erosion due to the impurities contained by fossil fuels.
Using a fossil fuel as the main energy source represents an input to the furnace of different kinds of heavy metals contained in the fuel, such as: vanadium pentoxide, iron oxide, chromium oxide, cobalt, etc. In the process of combustion most of the heavy metals evaporate because of the, low vapor pressure of the metal oxide and the high temperature of the melting furnace.
The chemical characteristic of the flue gases coming out of the furnace is mostly acid because of the high content of sulphur from the fossil fuel. Also the vanadium pentoxide presents an acid behavior such as the sulphur flue gases. Vanadium oxide is one of metals that represents a source of damage to basic refractories, because the acid behavior of this oxide in gaseous state. It is well known that the vanadium pentoxide strongly reacts with calcium oxide forming a dicalcium silicate at 1275 celsius degrees.
The dicalcium silicate continues the damage to form a phase of merwinite and the to monticelite and finally to forsterite, which reacting with vanadium pentoxide to form a low melting point of tricalcium vanadate.
The only way to reduce the damage caused to basic refractories is the reduction of the amount of calcium oxide in the main basic refractory in order to avoid the production of dicalcium silicate that continues reacting with vanadium pentoxide until the refractory may fail.
On the other hand, the main problem with the use of the petroleum coke is related with the high sulfur and vanadium content, which have a negative effect on the structure of the refractories in the furnaces. The foremost requirement characteristics of a refractory is to withstand exposure to elevated temperature for extended periods of time. In addition it must be able to withstand sudden changes in temperature, resist the erosive action of molten glass, the corrosive action of gases, and the abrasive forces of particles in the atmosphere.
The effect of the vanadium on the refractories has been studied in different the papers, i.e. Roy W. Brown and Karl H. Sandmeyer in the paper xe2x80x9cSodium Vanadate""s effect on superstructure refractoriesxe2x80x9d, Part I and Part II, The Glass Industry Magazine, November and December 1978 issues. In this paper the investigators tested different cast refractories which were centered on overcoming the vanadium attack in the flowing cast compositions, such as alumina-zirconia-silica (AZS), alpha-beta alumina, alpha alumina and beta alumina, which are commonly used in glass tank superstructures.
J. R. Mclaren and H. M. Richardson in the paper xe2x80x9cThe action of Vanadium Pentoxide on Aluminum Silicate Refractoriesxe2x80x9d describe a series of experiments in which cone deformation were carried out on sets of ground samples from bricks with alumina content of 73%, 42% and 9%, each sample containing admixtures of vanadium pentoxide, alone or in combination with sodium oxide or calcium oxide.
The discussion of the results were focused on the action of Vanadium Pentoxide, the action of Vanadium Pentoxide with Sodium Oxide and the Action of Vanadium Pentoxide with Calcium oxide. They concluded that:
1.xe2x80x94 Mullite resisted the action of vanadium pentoxide at temperatures up to 1700xc2x0 C.
2.xe2x80x94 No evidence was found of the formation of crystalline compounds or solid solutions of vanadium pentoxide and alumina or of vanadium pentoxide and silica.
3.xe2x80x94 Vanadium pentoxide may act as a mineralizer during the slagging of alumino-silicate refractories by oil ash, but it is not a major slagging agent.
4.xe2x80x94 Low-melting compounds are formed between vanadium pentoxide and sodium or calcium oxides, specially the former.
5.xe2x80x94 In reactions between either sodium or calcium vanadates and aluminosilicates, lower-melting-point slags are formed with bricks high in silica than with bricks highs in alumina.
T. S. Busby and M. Carter in the paper xe2x80x9cThe effect of SO3, Na2SO4 and V2O5 on the bonding minerals of basic refractoriesxe2x80x9d, Glass Technology Vol. 20, No. April, 1979, tested a number of spinels and silicates, the bond minerals of basic refractories, in a sulphurous atmosphere between 600 and 1400xc2x0 C., both with and without additions of Na2SO4 and V2O5. It was found that some MgO or CaO in these minerals was converted to the sulfate. The reaction rate was increased by the presence of Na2SO4 or V2O5. Their results indicate that the CaO and MgO in basic refractories can be converted to the sulfate if they are used in a furnace where sulphur is present in the waste gases. The formation of calcium sulfate occurs below 1400xc2x0 C. and that of magnesium sulfate below about 1100xc2x0 C.
However, as was described of the above, the effect of the vanadium on the refractories produce a great amount of problems in the glass furnaces, which has not solved in its totallity
Another problem of the use of the petroleum coke is related with the environment. The high content of sulphur and metals as nickel and vanadium produced by the combustion of the petroleum coke have provoked environmental problems. However, already exist developments for reduce or desulphurate the petroleum coke with a high content of sulphur (over 5% by weight). For example, the U.S. Pat. No. 4,389,388 issued to Charles P. Go forth on Jun. 21, 1983, concerns to the desulfurization of petroleum coke. Petroleum coke is processed to reduce the sulfur content. Ground coke is contacted with hot hydrogen, under pressurized conditions, for a residence time of about 2 to 60 seconds. The desulfurized coke is suitable for metallurgical or electrode uses.
The U.S. Pat. No. 4,857,284 issued to Rolf Hauk on Aug. 15, 1989, is related to a Process for removing sulphur from the waste gas of a reduction shaft furnace. In this patent it is described a novel process for removing the sulphur contained in a gaseous compound by absorbtion from at least part of the waste gas of a reduction shaft furnace for iron ore. The waste gas is initially cleaned in a scrubber and cooled, followed by desulphurizabon, during which the sulphur absorbing material is constituted by part of the sponge iron produced in the reduction shaft furnace. Desulfurization advantageously takes place at a temperature in the range 30xc2x0 C. to 60xc2x0 C. It is preferably carried out on the CO2 separated from the blast furnace gas and the blast furnace gas part used as export gas.
The U.S. Pat. No. 4,894,122 issued to Arturo Lazcano-Navarro, et al, on Jan. 16, 1990, is related to a process for the desulphurization of residuals of petroleum distillation in the form of coke particles having an initial sulphur content greater than about 5% by weight. Desulphurization is effected by means of a continuous electrothermal process based on a plurality of sequentially connected fluidized beds into which the coke particles are successively introduced. The necessary heat generation to desulphurize the coke particles is obtained by using the coke particles as an electrical resistance in each fluidized bed by providing a pair of electrodes that extend into the fluidized coke particles and passing an electrical current through the electrodes and through the fluidized coke particles. A last fluidized bed without electrodes is provided for cooling the desulphurized coke particles after the sulphur level has been reduced to less than about 1% by weight.
The U.S. Pat. No. 5,259,864 issued to Richard B. Greenwalt on Nov. 9, 1993, is related to a method for both disposing of an environmentally undesirable material comprising petroleum coke and the sulfur and heavy metals contained therein and of providing fuel for a process of making molten iron or steel preproducts and reduction gas in a melter gasifier having an upper fuel charging end, a reduction gas discharging end, a lower molten metal and slag collection end, and means providing an entry for charging ferrous material into the melter gasifier; introducing petroleum coke into the melter gasifier at the upper fuel charging end; blowing oxygen-containing gas into the petroleum coke to form at least a first fluidized bed of coke particles from the petroleum coke; introducing ferrous material into the melter gasifier through the entry means, reacting petroleum coke, oxygen and particulate ferrous material to combust the major portion of the petroleum coke to produce reduction gas and molten iron or steel preproducts containing heavy metals freed from combustion of the petroleum coke and a slag containing sulfur freed from combustion of the petroleum coke.
An additional factor to be considered in the glass industry is the control of the environment mainly the air pollution. The melting furnace contributes over 99% of both particulates and gaseous pollutants of the total emissions from a glass plant. The fuel waste gas from glass melting furnaces consists mainly of carbon dioxide, nitrogen, water vapor, sulphur oxides and nitrogen oxides. The waste gases released from melting furnaces consist mainly of combustion gases generated by fuels and of gases arising from the melting of the batch, which in turn depends on chemical reactions taking place within this time. The proportion of batch gases from exclusively flame-heated furnaces represents 3 to 5% of the total gas volume.
The proportion of the air-polluting components in the fuel waste gas depends on the type of the firing fuel, its heating value, the combustion air temperature, the burner design, the flame configuration, and the excess of air supply. The sulphur oxides in the waste gases of glass melting furnaces originated from the fuel used, as well as from the molten batches.
Various mechanisms have been proposed that include volatilization of these metal oxides and as hydroxides. Whatever the case, it is well known as the result of chemical analysis of the actual particulate matter, that more than 70% of the materials are sodium compounds, about 10% to 15% are calcium compounds, and the balance are mostly magnesium, iron, silica and alumina.
Another important considerations in the glass melting furnace is the emission of SO2. The emission of SO2 is a function of the sulfur introduced in the raw materials and fuel. During the time of furnace heating such as after a rise in production level, an abundance of SO2 is given off. The emissions rate of SO2 ranges from about 2.5 pounds per ton of glass melted to up to 5 pounds per ton. The concentration of SO2 in the exhaust is generally in the 100 to 300 ppm range for melting with natural gas. While using high sulfur fuel, approximately 4 pounds of SO2 per ton of glass for every 1% of sulfur in the fuel is added.
On the other hand, the formation of NOx as result of combustion processes has been studied and described by a number of authors (Zeldovich, J. The oxidation of Nitrogen in Combustion and explosions. Acta. Physiochem. 21 (4) 1946; Edwards, J. B. Combustion: The formation and emissions of trace species. Ann Arbor Science Publishers, 1974. p-39). These were recognized and by the Emissions Standards Division, Office of Air Quality Planning and Standards, USEPA, in their report on xe2x80x9cNOx Emissions from glass manufacturingxe2x80x9d include Zeldovich on homogeneous NOx formation and Edwards with his presentation of empirical equations. Zeldovich developed rate constants for the formation of NO and NO2 as the result of high temperature combustion processes.
Finally under normal operating condition, where flames are adjusted properly and the furnace is not starved for combustible air, very little CO or other residuals from incomplete combustion of fossil fuel are found in the exhaust. The gas concentration of these species will be less than 100 ppm, probably lower than 50 ppm, with a production rate of less than 0.2%/ton. The control for these pollutants is simply a proper combustion set up.
Processing techniques for the reduction of gaseous emissions are essentially restricted to the proper selection of firing fuels and raw materials, as well as to furnace design and operation. The U.S. Pat. No. 5,053,210 issued to Michael Buxel et al, on Oct. 1, 1991, describes a method and apparatus for the purification of flue gases, particularly for the desulphurization of and NOx -elimination from flue gas by multistage adsorption and catalytic reaction in gravity-flow moving beds of granular, carbon-bearing materials contacted by a transverse steam of the gas, in which a minimum of two moving beds are arranged in series with reference to the gas route so that NOx-elimination takes place in the second or any downstream moving bed. Where large volumes of flue gas from industrial furnaces must be purified, purification is adversely affected by the formation of gas streaks with widely varying sulphur dioxide concentrations. This disadvantage is eliminated in that the prepurified flue gas leaving the first moving bed and having a locally variable sulphur dioxide concentration gradient is subjected to repeated mixing before ammonia is added as reactant for NOx-elimination.
The U.S. Pat. No. 5,636,240 issued to Jeng-Syan et al, on Jun. 3, 1997, is related to an air pollution control process and apparatus for glass furnaces for use in the furnace""s waste gas outlet including passing the waste gases through a spray type neutralization tower to remove sulphates in the waste gases by spraying an absorbent (NaOH) to reduce the opacity of exhaust gas, and employing a pneumatic powder feeding device to feed flyash or calcium hydroxide periodically in a path between the spray type neutralization tower land a bag house to maintain normal functioning of the filter bag in the bag house.
Considering all the above, the U.S. patent application Ser. No. 09/816254 of the same applicant of the present invention is related to a method and""system for feeding and for burning pulverized petroleum coke in a glass melting furnace. In said invention, pulverized petroleum coke of type composed of carbon, sulfur, nitrogen, vanadium, iron and nickel is burned in a glass melting furnace of side-port type for melting glass raw materials for the manufacture of glass sheets or containers. Means are provided for supplying the pulverized fuel in at least a burner that is arranged by each one of a plurality of first and second side ports of a glass melting region of said glass melting furnace, for burning the pulverized fuel during cycles of melting glass, said glass melting furnace including refractory means at regenerative chambers of a glass melting furnace for resisting the erosive action of the melting glass, the corrosive action of combustion gases and the abrasive forces of particles in the atmosphere provoked by the burning of said pulverized fuel in the furnace. Finally, means for controlling the air pollution in a waste gas outlet after that the combustion of the pulverized fuel in the glass melting furnace has been carried out, said means for controlling the air pollution reducing the emissions of sulfur, nitrogen vanadium, iron and nickel compounds at the atmosphere.
However, in order to control the feeding and the burning of the pulverized fuel in the glass melting furnace during cycles of melting glass, the present invention is related to a control system for controlling the feeding and burning of said pulverized fuel in the glass melting furnace. The control is done through a continuous monitoring of the different variables or parameters involved in the process such as, gas flow in each burner, pulverized fuel feeding velo city; blower velocity; blower pressure; transportation air flow; furnace internal pressure; gases extraction velocity in the chimney; gases pressure in the exit and entrance of the environment control system; furnace temperature; temperature in the combustion chambers; and the temperature profile in the furnace, in order to analyze the behavior of the furnace, when petroleum coke is used. All these factors are monitored and synchronized by means of a programmable controller in order to carried out the combustion air and exhaust heat cycles in the furnace, which are alternate every 20 minutes, or 30 minutes, depending on the specific furnaces. The automatic cycles also can be carried out by means for the monitoring of the temperature in the furnace, in accordance at preestablished temperature cycles in the programmable controller.
In accordance with the present invention is a first objective of the present invention to provide a control system for feeding pulverized fuel in a glass melting furnace, which automatically controls the feeding of the pulverized fuel in the furnace.
Is another objective of the present invention, to provide a control system for feeding pulverized fuel in a glass melting furnace, which monitors and controls all the variables of the glass melting process in order to carried out a synchronized operation of the fuel, the combustion air and the exhaust gases cycles in the glass furnace.
Another objective of the present invention is to provide a control system for feeding pulverized fuel in a glass melting furnace, which allows a coordinated operation sequence among all different systems that interact along the whole process of storing, handling, transporting, feeding, burning and disposing of the pulverized fuel in the glass melting process. The interaction of control system takes place between the combustion control system with a feeding control system, a melter control system, an environmental control system, a waste handling control system, and a storage and transportation control system, in order to carry out the data interchange of all the variables manipulated and detected in the different stages of the process and equipments and for feeding the pulverized fuel to a plurality of burners that are associated with a pair of sealed regenerators of a regenerative glass melting furnace.
Another objective of the present invention is to provide a control, system for feeding pulverized fuel in a glass melting furnace, wherein the burners can be arranged in the melting chamber for burning petroleum coke, gas or fuel oil.
These and other objectives and disadvantages of the present invention will be evident to the experts in the field from the following detailed description of the invention, which is illustrated in the attached drawings.